Gleipnir

In Norse mythology, Gleipnir (Old Norse "open one"[1]) is the binding that holds the mighty wolf Fenrir (as attested in chapter 34 of the Prose Edda book Gylfaginning). The Gods had attempted to bind Fenrir twice before with huge chains of metal, but Fenrir was able to break free both times. Therefore, they commissioned the dwarves to forge a chain that was impossible to break. To create a chain to achieve the impossible, the dwarves fashioned the chain out of six supposedly impossible things:

Therefore, even though Gleipnir is as thin as a silkenribbon, it is stronger than any ironchain. It was forged by the dwarves in their underground realm of Niðavellir.

Gleipnir, having bound Fenrir securely, was the cause of Týr's lost hand, for Fenrir bit it off in revenge when he was not freed. Gleipnir is said to hold until Ragnarök, when Fenrir will finally break free and devour Odin.